In the retorting of crushed oil shale with hot eduction gases, a persistent problem has been that of providing efficient retorting of shale particles varying widely in size. Commonly applied shale crushing techniques are controlled to produce a consist containing no particles larger than a permissible maximum, usually about 2 to 4 inches in diameter. Such crushing also inherently produces from about 2 to 15 weight percent of shale "fines" having a diameter of less than about 1/4 inch. If this entire consist is utilized in the most prevalent of retorting methods, i.e. countercurrent flow of hot retorting gas through a compact moving bed of the shale particles, serious problems arise. Among these problems are high gas pressure drop through the bed, uneven distribution of gas flow in the bed and increased solids content in the product oil.
To avoid these problems, it has become customary in the art to screen out the fines from the retort feed, and either subject them to separate retorting, or discard them. It would obviously be economically attractive to provide some means for integrating the retorting of shale fines with the retorting of the main body of shale, if the aforementioned problems could be avoided.
U.S. Pat. No. 3,004,898 discloses a solids-upflow, gas-downflow retorting system in which raw shale fines of 0 to 1/2 inch in diameter, and amounting to about 20 weight percent of the total shale feed, are subjected to separate retorting in concurrent flow with eduction gas, while a very minor proportion of shale fines in the micron size range recovered as a concentrated sludge in the product oil is introduced onto the surface of the spent shale at the top of the retort, where the oil content of the sludge is vaporized and the remaining "micro"-fines are instantaneously retorted. A scraper at the top of the retort continuously scrapes spent shale from the retort maintaining a flat upper surface thereof. Agitation provided by the scraper specifically disclosed in said patent tends to increase the downward infiltration of micro-particles of spent shale to increase bed pressure drop and contaminate the product oil.
My copending application Ser. No. 891,280 discloses a retorting method and apparatus in which the upwardly flowing shale is allowed to form a free-standing cone above the top edge of the retort vessel, and raw shale fines are distributed on the sloping surface of the cone of shale, at or near the apex of the cone. The residence time available for retorting of the raw shale fines is determined by the rate of free gravitation of shale down the side of the cone. Due to the expected variance in size distribution of the raw shale fines from time to time in a commercial-scale retorting operation, this residence time may not always be the optimum. Accordingly it is desirable to have a retorting apparatus and method which allows for the independent control of the raw shale fines residence time in the retort.